Hi everyone. I have still no been able to figure this out and need a little help. </P>
I have had constant hum-related problems with my Gulbrasnen Model D. I had all the filter capacitors replaced not once, but twice, by two different people (there are two power supplies - one for console power, the other for amp power - this is all solid state). I have tried putting a scope on here several times and I was not reading much ripple current at all; on one of the supplies there was a tiny bit of AC and I just wired in a new cap in parallel and that reduced the hum. I can get the hum quieter but not gone. Then, days later, there will be hum again. It sounds more like 60Hz than 120Hz. Here are some weird things and maybe you can make heads or tails out of it:</P>
1. Since the second rebuild I noticed that when I turn the organ off, a louder "buzzing" sound is produced, then slowly fades away. It sounds like something is being discharged. The only thing that made me curious was that two 1500uF caps in the amp PS unit were replaced with 5000uF caps, but I was told that was OK. </P>
2. The hum does not get louder when I turn up the expression pedal. Strangely enough, the hum is actually more noticable when the expression pedal is all the way down (quietest).</P>
3. My organ has a switched AC receptacle. If I plug certain things into this receptacle (like my rhythm box), the hum gets louder, even if I do not have therhythmbox powered on. But, then I tried plugging a simple lamp into it and the hum did not get louder.</P>
4. The transformer on my amp power supply does acoustically buzz (loose plates??). The speaker lines do plug directly into the amp power supply. Could this make a difference? FYI the amp power supply supplies +34 and -34 to the amp boards and also acts as a speaker output box.</P>
5. If I disconnect the line inputs into the 2 solid state amplifier boards (it is a 2-channel organ), the hum is reduced significantly. </P>
6. In several areas of the audio processing circuitry (voicing, pre-amps, etc) I can induce a loud hum by passing my hand above certain components. Once I touch the component(s) this hum goes away. As long as I keep a finger on the component, I can pass my other hand over the same components without introducing any additional hum.</P>
7. On the amp power supply, I noticed once that the transformer was buzzing louder than normal and that the hum out of the speakers was louder as well. I had also started getting some "rumbling" out of the amplifiers and one would intermittently be dead completely. I opened up the PS and discovered that the -34 lead on the filter caplooked "burned" and the solder joint was loose/cold. On the amp boards themselves, the 150-ohm resistors on the -34 power line were fried. I replaced the resistors in the amp boards and re-soldered the -34 supply line on the amp power supply. Then the hum got better. But,I don't know what would have caused this - could the solder joint have been bad all along, or could something have heated it up? Again, this is one of those 5000uF caps that replaced the original 1500uF caps.</P>
8. Seemingly be design, the speaker leads run very close to several AC lines - there are AC lines for the leslie motor, the incoming power (connects to a twisted AC line), the amp power supply itself, etc. There does not appear to be andshielding in place. Could this be introducing hum into the speakers?</P>
I don't know what to make of all this -maybe there are several problems going on. I thought I'd throw it all out there and see if you have any ideas. Thanks again for all your help.</P>
-jim</P>
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